In the stem of monocot there exist three tissue systems —dermal, vascular and ground tissues, which consist of epidermis, vascular bundles and all tissues except the former two respectively. A brief account of the arrangement and structure is given below.
Epidermis:
Epidermis is more or less like dicots. In contrast to dicot stem, the epidermis is usually devoid of glandular and non-glandular trichomes.
Ground Tissue:
In monocotyledonous stems all tissues except vascular bundles present internal to epidermis are described as ground tissue because there exists no real distinction between cortex and pith. In many genera of Liliaceae (ex. Asparagus, Fig. 31.12) and Amaryllidaceae there is differentiation of cortex and stele with distinct endodermis and pericycle.
Peripheral layers of ground tissue, i.e. cells beneath the epidermis may be composed of sclerified parenchyma or sclerenchyma. Sclerenchyma, in the form of a continuous ring, composes the pericycle of Asparagus. The other portions of ground tissue are composed of parenchyma upon which the vascular bundles appear to be embedded.
Vascular bundles in monocot stem usually lie scattered throughout the ground tissue and this type of distribution is termed as atactostele. In many genera like Coix, Triticum, Oryza etc. they are arranged more or less in a ring.
Each vascular bundle may be leptocentric or amphivasal (ex. Dracaena, Aloe etc.) or collateral and closed, i.e. cambium is absent (ex. Zea, Asparagus etc.). In collateral bundles the xylem are arranged in the form of capital ‘Y’ or ‘V’ or ‘U’, which appear in cross section.
Large metaxylem vessel elements form the arms of the alphabets and the protoxylem composes the rest. In some genera protoxylem disintegrates and the cavity thus formed is termed as protoxylem cavity or lacuna or canal.
Phloem groups are situated in the grooves of Y or V or U-shaped xylem, i.e. xylem encircles phloem on the three sides. A ring of sclerenchyma termed bundle sheath usually encircles each vascular bundle. In Asparagus the bundle sheath is absent.